1. Field of the Invention
This invention relates generally to communication systems, and more particularly to systems having many data terminals transmitting data under controlled access to a transmission medium.
2. Description of the Related Art
In a communication system, many users must be allocated transmitting time to transfer their data or voice over a transmission medium. The transmission medium comprises a communication channel divided into time slots that are allocated among the users by a central controller. The time slots can be allocated on the basis of a contention protocol procedure, which is similar to a first-come, first-served rule, where users are granted time as they request it. If a user requests the channel and it is not available (i.e., the channel is in use), then the user must wait for a time interval and attempt again. As the system becomes active, two users might attempt to transmit over the channel at the same time, and a data collision will occur. This forces the users to retransmit or wait for a time interval before transmitting again, and may result in the users finding the channel unavailable when again attempting to transmit.
As an alternative to a contention protocol, users can be allocated time slots on the basis of a polling protocol procedure, in which all users are guaranteed access to the channel on a regular basis, each user being polled, or granted transmitting time, in sequence. Each user who requires transmission time must wait its turn. Under a polling protocol, a user might be forced to wait for its turn even if no other user in the system has data to transmit, because the other users are being polled. Each user must wait until it is polled to indicate whether or not it has data for transmission. A certain amount of such waiting time is inevitable under a polling protocol, and is referred to as the polling overhead of a polling protocol procedure. Those skilled in the art will appreciate that a contention protocol is efficient for lightly-loaded systems, or networks, and is inefficient for busy networks, while a polling protocol works efficiently with crowded, busy networks but is inefficient with lightly-loaded networks.
It is known to provide communication systems with hybrid protocol procedures, in which users are divided into groups and the groups are polled, while the users within a polled group then decide who among them transmits on the basis of a contention protocol. In this way, groups of users are guaranteed transmitting time on a regular basis. Dividing the users into polled groups reduces the amount of polling overhead and risk of data collision. The assignment of users to particular groups, and the size of the groups, is a compromise based on trade-offs between the polling overhead and the contention waiting time or rate of data collisions.
Many communication systems experience fluctuating usage rates, in which the system is lightly loaded for certain intervals of time, in between which the system becomes heavily loaded with many users seeking to transmit. For such systems, neither the polling protocol nor contention protocol is optimal. Hybrid protocol systems can be an improvement, but often the need for access to the network will fluctuate in unpredictable ways, or the distribution of users requiring access to the transmission medium at any one time can change throughout the network, making the assignment of users to groups and the size of the groups not optimal.
It should therefore be appreciated that there is a need for a communication system that capitalizes on the advantages of a contention protocol, when the network is lightly loaded, and of a polling protocol, when the network is crowded and busy. The present invention satisfies this need.